Abstract: Germanium is an important infrared optical material. To reduce Fresnel reflection loss on the germanium surface and improve the light utilization rate, the anti-reflection performance of the conical microstructure on a germanium substrate was studied. Based on the finite difference time domain (FDTD) method and the single factor method, the effects of the microstructure parameters, such as duty ratio, period, height, and the angle of incidence on reflectivity are discussed for the 8 μm to 12 μm long-wavelength infrared band. The structural parameters of the microstructure at low reflection was determined. Its average reflectivity over the entire wavelength range is less than 1%, which is much lower than the 35.47% reflectivity of the slab germanium structure, and the reflectivity in the wavelength range of 9 μm to 11 μm is less than 0.5%. The average reflectivity of the conical microstructure remained low when light was incident at 40°. By comparing the optimized conical microstructure with the slab structure, the excellent antireflection performance of the conical microstructure over the entire wavelength range was further confirmed based on the equivalent refractive index, reflected electric field intensity distribution, and absorption per unit volume.